Hitherto, a linear actuator, which converts an electric input signal into a linear movement of a moving member and outputs the same, mainly includes a guide mechanism for linearly guiding the moving member while bearing an external force, and a drive mechanism for causing the moving member to advance and retreat along the guide mechanism. There are various known examples of the drive mechanism, including a linear actuator which uses a hydraulic cylinder or an air cylinder as the drive source, and which transmits a linear reciprocating motion of a piston as it is to the moving member, and an actuator which uses a motor as the drive source and which converts a rotary motion of the motor to a linear motion by using a ball screw. Further, a linear actuator is also known which uses as the drive source a linear motor including a stator and a moving element and which transmits a linear reciprocating motion of the linear motor as it is to the moving member.
While there are various combinations of the drive mechanism and the guide mechanism according to the requisite thrust for the moving member, load bearing capacity, moving speed, precision, etc., simply combining an arbitrary drive mechanism and guide mechanism results in a linear actuator of a rather large size, which leads to a rather poor usability.
JP 2003-278873 A discloses a linear actuator of a relatively compact structure in which the drive mechanism is contained in the guide mechanism. In this linear actuator, a ball screw made up of a screw shaft and a ball nut are accommodated in a cylindrical housing having a hollow part, and the screw shaft is rotatably supported within the hollow part. A motor is fixed to one end of the cylindrical housing, and it is possible to impart an arbitrary rotation to the screw shaft by this motor. Further, an axially extending spline groove is formed in the outer peripheral surface of the housing; a cylindrical moving member is fit-engaged with the outer side of the housing through the intermediation of the spline groove, and the cylindrical moving member is freely movable in the axial direction of the housing while prevented from rotation relative to the housing. Further, the housing has a slit-like opening extending in the axial direction, and the ball nut of the ball screw is connected with the moving member through the intermediation of this opening.
In the actuator constructed as described above, when the motor is rotated to impart an arbitrary rotation to the screw shaft contained in the hollow part of the housing, the ball nut moves within the hollow part upon the rotation of the screw shaft, and, on the outer side of the housing, the cylindrical moving member moves in the axial direction together with the ball nut. Thus, it is possible to move the cylindrical moving member along the axis of the housing in an arbitrary direction and by an arbitrary amount according to the rotating direction and the rotating amount of the motor, respectively.
Patent Document 1: JP 2003-278873 A